Photochemical Oxidation of Hydrocarbons by Nitropyridinium Salts

Full text of DOI:10.1021/jo971617a

1138
J . Org. Chem. 1998, 63, 1138-1143
P h otoch em ica l Oxid a tion of Hyd r oca r bon s by Nitr op yr id in iu m
Sa lts
Stephan Negele, Katja Wieser, and Theodor Severin*
Institut fu¨r Pharmazie und Lebensmittelchemie der Ludwig-Maximilians-Universita¨t Mu¨nchen,
Sophienstrasse 10, D-80333 Mu¨nchen, Germany
Received September 2, 1997
Aliphatic and alicyclic hydrocarbon compounds can be
oxidized by photoactivated 4-nitropyridinium salts (1).
Reaction of 1 with indan, tetralin, 2-ethylnaphthalene,
or cyclohexane leads to the formation of indanone,
tetralone, 2-acetylnaphthalene, cyclohexanol, and cyclo-
hexanone, respectively. Alkyl aryl ketones and dialkyl
ketones are transformed in a regiospecific reaction into
1,4-dicarbonyl compounds. Isoamyl acetate is oxidized
to give the 3-hydroxy derivative as sole product. In
contrast to the compounds mentioned so far, tetrahydro-
furan reacts with nitrobenzene as well as with photoac-
tivated pyridinium salts to afford butyrolactone and
octahydro-2,2′-bifuranyl. When acetone is applied as
cosolvent, a tetrahydrofuran-acetone derivative is formed
as main product.
F igu r e 1. Nitropyridinium salts 1a -c.
Photochemical reactions of aromatic nitro compounds
have been intensively investigated.1 Several papers deal
with the radical anion formation of aromatic nitro
compounds or with hydrogen abstraction reactions which
ultimately lead to the reduction of the nitro group. For
instance, nitrobenzene is photoreduced by 2-propanol,
and the major products under neutral conditions are
acetone and phenylhydroxylamine.2 Ortho-substituted
nitrobenzenes can undergo intramolecular hydrogen-
transfer reactions, in which the nitro group is reduced
to a nitroso function while an oxygen atom is inserted
into a CH-bond located in the ortho position.3 This type
of reaction has led to the development of the o-nitrobenzyl
residue as a protecting group for alcohols, amines, and
carboxylic acids. p-Methoxy-substituted alkyl benzenes
are oxidized at the benzylic CH-bonds by nitrobenzene
under irradiation in good yields, whereas ethylbenzene
and cyclohexylbenzene did not react under comparable
conditions.4 In a few cases it has been shown that a
nitrobenzene moiety attached to a steroid molecule can
oxidize the rigid hydrocarbon at a remote position after
photochemical excitation, but the yields are low.5 Ir-
radiation of substituted nitrobenzenes in triethylamine
has been reported to afford acetaldehyde, which may
F igu r e 2. Photooxidation of indan 2a and tetralin 2b.
react further with the reduction product of the nitro
aromatic compound.6
Because electron-transfer processes are essential in
photochemical reactions of aromatic nitro compounds, we
argued that nitropyridinium salts might be very reactive.
Here we report that hydrocarbon chains with different
types of substituents can be oxidized by photoactivated
nitropyridinium salts.
Resu lts
Initial experiments indicated that activated and un-
activated aliphatic hydrocarbons can be oxidized by
aromatic nitro compounds under irradiation, but the
yields are generally low. When we compared different
aromatic and heteroaromatic nitro compounds, it turned
out that 4-nitropyridinium salts are far more reactive
than aromatic nitro compounds such as nitrobenzene or
1,3-dinitrobenzene. The nitropyridinium salts 1a -c
applied in this investigation are easily obtained by
alkylation of 4-nitropyridine or 4-nitropyridine N-oxide
with trimethyl- or triethyloxonium tetrafluoroborate,
respectively (Figure 1).
When equimolar amounts of indan 2a and the nitro-
pyridinium salt 1c in a solution of acetone are irradiated
by a HPK 125 W lamp equipped with a Pyrex filter for 5
h at room temperature, ketone 3a is produced in a 72%
yield. Similary tetralin 2b reacts to give ketone 3b in
71% yield (Figure 2) and 2-acetylnaphthalene 5 is
obtained from 2-ethylnaphthalene 4 in a 12% yield after
12 h irradiation (Figure 3). In accordance to expectation,
a methylene group attached to an aromatic nucleus is
activated with respect to this type of photochemical
oxidation.
* To whom correspondence should be addressed. Tel: 0049/89/5902-
387. Fax: 0049/89/5902-447. E-mail: kmw@IRIS.pharm-chem.phar-
mazie.uni-muenchen.
(1) (a) Chow, Y. L. In The Chemistry of amino, nitroso and nitro
compounds and their derivatives (The Chemistry of functional groups.
Supplement F); Patai, S., Ed.; Wiley: New York 1982; Vol. 1, pp 181-
281. (b) Morrison, H. A. In The Chemistry of nitro and nitroso groups
(The Chemistry of functional groups); Feuer, H., Ed.; Patai, S., Series
Ed.; Wiley: New York, 1969; Vol. 1, pp 165-212.
(2) Hurley, R.; Testa, A. C. J . Am. Chem. Soc. 1966, 88, 4330.
(3) (a) Literature 1a, p 198 and literature cited, therein. (b)
Literature 1b, p 185 and literature cited therein.
(4) Libman, J . J . Chem. Soc., Chem. Commun. 1977, 868.
(5) Scholl, P. C.; Van De Mark, M. R. J . Org. Chem. 1973, 38, 2376.
(6) Literature 1a, p 190 and literature cited therein.
S0022-3263(97)01617-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/23/1998

Photooxidation of Hydrocarbons by Nitropyridinium Salts
J . Org. Chem., Vol. 63, No. 4, 1998 1139
F igu r e 3. Photochemical oxidation of 2-ethylnaphthalene 4.
F igu r e 5. Photooxidation of ketone 8.
F igu r e 6. Photooxidation of the aliphatic ketone 10.
F igu r e 4. Oxidation products of ketones 6a -c.
Ta ble 1. Va r ia tion in Yield for F or m a tion of Dik eton es
7a -c, 9 w ith Nitr o Com p ou n d s
nitro
compound
diketones
(% yieldb)
entry
substrate
conditionsa
F igu r e 7. Photochemical oxidation of cyclohexane 12.
1
2
3
4
5
6
6b
6b
6c
8
6a
6a
1a
1b
1b
1b
1a
1a
a, 5h
a, 5h
a, 5h
a, 5h
a, 5h
b, 5h
7b (39)
7b (63)
7c (60)
9 (79)
7a (7)
7a (11)
a
F igu r e 8. Photooxidation of ester 16.
a: 10 mM ketone, 10 mM nitropyridinium salt, acetone. b:
b
10 mM ketone, 10 mM nitropyridinium salt, acetonitrile. Yields
determined by GC-MS.
of 5 mL of 2-hexanone and 15 mL of acetonitrile for 5 h
affords 0.02 mmol of diketone 11 (Figure 6). Based on
the assumption that the nitro compound is reduced to a
hydroxylamine moiety, the yield is 8%. Other products
detectable by GC can be neglected.
Even unactivated hydrocarbon compounds can be
oxidized by photoactivated nitropyridinium salts. Dif-
ficulties arise when the molecule contains different CH-
groups of comparable reactivity or when the oxidation
product may react further. The problem is well-known
from the transformation of cyclohexane into cyclohex-
anone by metal-catalyzed oxidation, a reaction which has
been intensively investigated.
Irradiation of a solution of 1 mmol of 1b in a mixture
of 30 mL of cyclohexane and 70 mL of acetone for 5 h
leads to the formation of 0.32 mmol of cyclohexanone and
0.82 mmol of cyclohexanol, besides small amounts of
bicyclohexyl (Figure 7). Other reaction products of
cyclohexane have not been detected by GC-MS analysis.
Similar irradiation of 1 mmol of 1c in a solvent of
isoamyl acetate and acetone provides 1.3 mmol of 3-hy-
droxyisoamyl acetate. The product was identified as
nitrobenzoyl derivative by spectroscopic data. Obviously
oxidation of the tertiary carbon atom is the preferred
reaction (Figure 8).
In contrast to the compounds discussed so far, photo-
chemical oxidation of tetrahydrofuran cannot only be
achieved by action of activated nitropyridinium salts, but
with less reactive nitroaromatic compounds as well.
Irradiation of 0.2 mol of nitrobenzene in a solution of 5
mL of tetrahydrofuran and 15 mL of acetonitrile leads
to the formation of butyrolactone (0.07 mol, 34% yield)
and octahydro-2,2′-bifuranyl (0.2 mol, 48% yield, mixture
of stereoisomers) as main products (Figure 9). Other
compounds detectable by GC can be neglected. On the
other hand, irradiation of nitrobenzene in a solvent
mixture of tetrahydrofuran and acetone affords the
adduct 21 as main product besides lactone 19 and
Irradiation of equimolar amounts of 1-phenylpen-
tanone 6b and the salt 1b in acetone, under the condi-
tions described above, leads to the formation of the
diketone 7b in 63% yield. Besides 7b only small amounts
of a cyclobutanol derivative, resulting from 1,4-biradical
photocyclization, and acetophenone can be detected by
GC. The same diketone 7b is also obtained, when the
nitropyridinium salt 1a is applied as oxidizing agent, but
the yields are lower (39%). Other examples, which
demonstrate the high regioselectivity of this type of
photochemical oxidation, are shown in Figure 4 or 5
respectively. 1-Phenylhexanone 6c and 2-propyltetralone
8 are oxidized to afford the diketones 7c and 9 in high
yields. In contrast, oxidation of 1-phenylbutanone 6a ,
when irradiated with 1a in acetonitrile or acetone,
produced the keto aldehyde 7a in only modest yield
(entries 5 and 6, Table 1). Probably the aldehyde group
reacts further under irradiation, a process which is still
under investigation.
The reaction products were identified by comparison
(GC-MS) with commercially available substances or
isolated by flash chromatography and identified by
spectroscopic data. Quantification was achieved by GC
using internal standards.
Compared with the nitropyridinium salts 1a -c, aro-
matic nitro compounds are only poor oxidizing agents.
For instance: irradiation of equimolar amounts of ketone
6b and nitrobenzene under the conditions described for
the nitropyridinium salts, affords oxidation products in
yields below 1%.
Aliphatic ketones with two alkyl side chains are not
as reactive as the alkyl aryl ketones discussed so far.
Nevertheless, the expected oxidation product can be
obtained in moderate yield, when the ketone is applied
in high molar ratio with respect to the nitropyridinium
salt. For example irradiation of 0.2 mmol 1b in a mixture

1140 J . Org. Chem., Vol. 63, No. 4, 1998
Negele et al.
Sch em e 1
may lead to the formation of the radical cation 29. This
key step is substantiated by the results obtained by
anodic oxidation of alkyl ketones, a process which is
discussed below. It should be noted that electron ab-
straction from carboxylic acids by photoactivated acri-
dine, which ultimately leads to decarboxylation, is a well-
known process.7 In the photochemical oxidation of
ketones described in this paper, the ketone may have a
2-fold function: it could act as a sensitizer for the
activation of the nitro compound, but on the other hand
the ground-state carbonyl compound should react with
the activated nitropyridinium salt. The further reaction
of the radical cation 29 may proceed as follows: intramo-
lecular attack on a remote hydrogen-carbon bond by the
oxygen radical would afford the carbon radical 30, which
may transfer a proton to the pyridinium radical 26.
Oxidation of radical 30 by combination with the nitro-
pyridinium derived radical 23 and subsequent NO-bond
cleavage should proceed as described above for the
2-ethylnaphthalene radical 22.
The regioselective photochemical oxidation of alkyl aryl
ketones bears similarities to a long known electrochemi-
cal process. It has been demonstrated that anodic
oxidation of alkyl ketones can lead to a substitution at a
remote position.8 This type of reaction is exemplified by
the electrochemical oxidation of 2-hexanone in acetoni-
trile with lithium perchlorate as electrolyte, which re-
sulted in the formation of 5-acetamido-2-hexanone in 40%
yield (Scheme 3). Analogous reactions of other alkyl
ketones have been described, but the yields are only
modest (10-40%). The first steps of the reaction path-
way (10 ff 30) are analogous to the photochemical
oxidation discussed above.
F igu r e 9. Oxidation products of THF.
octahydro-2,2′-bifuranyl. The reaction products 19, 20,
and 21 have been isolated via flash chromatography and
identified by comparison of their MS- and NMR-spectral
data with literature values.
When the compound to be oxidized was applied in
excess, the yields are calculated on the basis that the
nitro group is reduced to a hydroxylamine function.
Otherwise the yields were referred to the starting
materials. Actually the process is more complex, and the
reduction products of the nitro compounds have not been
separated and analyzed in detail.
Discu ssion
The results obtained so far should be discussed under
mechanistic aspects. Hydrogen atom transfer or succes-
sive electron and proton transfer to the photoactivated
4-nitropyridinium salt (1) as the first steps of the reaction
would account for the formation of the photooxidation
products of the aromatic compounds with alkyl side
chains. In the formal scheme the reaction mechanism
is exemplified by the transformation of 2-ethylnaphtha-
lene into 2-acetylnaphthalene (Scheme 1). Reorganiza-
tion of the radical pair [22/23] may lead to an interme-
diate of structure 24, which should degrade to give the
nitroso compound 28 and the alcohol 27. Previously
photochemical oxidations of alcohols by nitro or nitroso
compounds have been carefully investigated and mecha-
nistically discussed.1,2
The regiospecific photochemical oxidation of alkyl aryl
ketones can be explained by a different reaction mech-
anism as well. It has been rigorously established that
irradiation of ketones with alkyl side chains can produce
biradicals of type 35, which can typically undergo 1,4-
cyclization to cyclobutanol derivatives or fragmentation
according to the Norrish Reaction as depicted in Scheme
A mechanism which would explain the regiospecific
oxidation of alkyl aryl ketones is depicted in Scheme 2.
In the first step an electron transfer from the ground-
state ketone to the photoactivated nitropyridinium salt
(7) Okada, K.; Okubo, K.; Oda, M. Tetrahedron Lett. 1992, 33, 83-
84.
(8) Becker, J . Y.; Byrd, L. R.; Miller, L. L.; So, Y.-H. J . Am. Chem.
Soc. 1975, 97, 853.

Photooxidation of Hydrocarbons by Nitropyridinium Salts
Sch em e 2
J . Org. Chem., Vol. 63, No. 4, 1998 1141
Sch em e 3
Sch em e 5
Sch em e 4
ester carbonyl group could be involved as has been
demonstrated for the ketones 6a -c, 8, and 10. In a
series of preliminary investigations, solutions of 1 in
aliphatic esters such as pentyl acetate and methyl
cyclohexanecarboxylate with the cosolvent acetone or
acetonitrile have been irradiated under standard condi-
tions. Analysis of the resulting reaction mixtures by GC-
MS showed that the alkyl chains are oxidized preferably
at the 3-position, but compared with the ketones 6a -c,
8, and 10 the regiospecifity is less pronounced. This type
of reaction is still under investigation.
4. During the photochemical reaction of ketones with
nitropyridinium salts, cyclobutanols and Norrish prod-
ucts are also formed, but only in a very low amount
compared to the oxidation products. The presence of the
nitropyridinium salt interferes with the normal behavior
of the biradicals and results in the formation of oxidation
products by quenching the biradical intermediate. This
electron transfer from 35 to the unactivated nitropyri-
dinium salt would lead to the same intermediate 30 as
proposed in the first reaction scheme. Proton transfer
of the oxonium cation to the nitro radical would afford a
species of type [31/23], and further oxygenation of the
carbon radical 31 should proceed as discussed above.
Even the unactivated hydrocarbon cyclohexane reacts
with the nitropyridinium salt 1b under irradiation to give
cyclohexanol and cyclohexanone. The cyclohexyl radical
must be considered as an intermediate which should
react further to provide cyclohexanol and cyclohexanone.
Remarkably bicyclohexyl is produced as byproduct in low
amount, which has been detected by GC-MS (not quanti-
fied). The comparatively high yield of oxidation products
with respect to the pyridinium salts suggests that not
only the nitro compound but also the nitrosopyridinium
salt is an oxidizing agent under the conditions employed.
The easy photochemical oxidation of a tertiary CH-
group is demonstrated by the transformation of 16 to the
keto alcohol 17 (Figure 8). It could be argued that the
Compared to the compounds discussed so far, electron
abstraction from aliphatic ethers is more easily achieved.
Irradiation of nitrobenzene in a solution of tetrahydro-
furan has been investigated under mechanistic aspects,
and the formation of a nitrobenzene radical anion and a
species resulting from hydrogen abstraction have been
discussed on the basis of esr spectral data.9 We inves-
tigated the formation of the oxidation products of tet-
rahydrofuran under a variety of conditions (Scheme 5).
Lactone 19 and octahydro-2,2′-bifuranyl 20 (mixture of
stereoisomers) are obtained as main products, when the
pyridinium salt 1b is irradiated in a solution of tetrahy-
drofuran and acetonitrile. The electron-donating prop-
erty of the cyclic ether is demonstrated by the fact that
20 and the lactone 19 are obtained as well by action of
photoactivated nitrobenzene as oxidizing agent. On the
other hand, in the presence of acetone the C,C-coupling
product 21 is obtained in high yield. This compound may
arise through addition of the tetrahydrofuranyl-radical
to the ketone, followed by a radical chain process. 21
has been synthesized previously by intramolecular cy-
(9) J anzen, E. G.; Gerlock, J . L. J . Am. Chem. Soc. 1969, 91, 3108.

1142 J . Org. Chem., Vol. 63, No. 4, 1998
Negele et al.
clization of 4,5-epoxy-5-methylhexan-1-ol.10 Octahydro-
2,2′-bifuranyl was identified as a reaction product of
tetrahydrofuran and tert-butyl hydroperoxide.11
Summarized, photoactivated 4-nitropyridinium salts
are effective reagents for the oxidation of hydrocarbon
chains.
the irradiation, the crude reaction mixture was concentrated
in vacuo. Unless otherwise noted, all products were identified
directly by comparing their retention times and mass or NMR
spectra with those of the pure authentic samples used as
reference compounds or those given in the literature. Quan-
tification was performed by gas chromatography using internal
standards as indicated.
1-Phenylpentanone (0.16 g, 1 mmol), 1-phenylhexanone
(0.25 g, 1 mmol), or 2-propyltetralone (0.19 g, 1 mmol),
respectively, were irradiated as a 10 mM solution in acetone
with 1b (0.25 g, 1 mmol). After 5 h, the solvent was
evaporated, and the crude reaction mixtures were purified via
flash chromatography to afford 7b (110 mg, 0.63 mmol, 63%),
7c (113 mg, 0.59 mmol, 60%), or 9 (160 mg, 0.79 mmol, 79%),
respectively. The isolated products were identified by com-
parison of their spectroscopic data with literature values.15,16
The yields were determined using cyclohexanedione as internal
standard, and only in the case of 2-propyltetralone was
1-phenylpentanone added. All yields were based on the
corresponding starting materials.
Exp er im en ta l Section
Ma ter ia ls a n d Meth od s. The following materials were
obtained from commercial sources and used as received:
acetophenone, butyrolactone, cyclohexane, cyclohexanedione,
cyclohexanol, cyclohexanone, ethylnaphthalene, 2-hexanone,
2,5-hexanedione, indan, indanone, isoamyl acetate, 2-methyl-
naphthyl ketone, 4-nitropyridine N-oxide, octahydro-2,2′-bi-
furanyl, 1-phenylbutanone, 1-phenylpentanone, tetralin, te-
tralone, triethyloxonium tetrafluoroborate, trimethyloxonium
tetrafluoroborate. THF and acetonitrile were purchased and
distilled from sodium/benzophenone ketyl or CaH2, respec-
tively, before use. Other solvents were used without further
purification. 4-Nitropyridine,12 1-phenylhexanone,13 and 2-pro-
pyltetralone14 were prepared by literature methods. Photolysis
was carried out with a HPK 125 W lamp from Philips.
Preparative column chromatography was performed with a
32-63 mesh silica gel using a mixture of ethyl acetate and
petroleum ether (1:4) as solvent system. Mass spectra were
recorded on a HP 5971 A mass selective detector interfaced
with a HP 5890 series II gas chromatograph equipped with a
25 m × 0.25 mm fused silica capillary column. 1H NMR
1-Phenylbutanone (0.15 g, 1 mmol) and 1a (0.23 g, 1 mmol)
were dissolved in 100 mL of acetonitrile and irradiated for 5
h. The reaction mixture was worked up and quantified as
indicated above affording keto aldehyde 7a (18 mg, 0.11 mmol,
17%).17,18
Indan (0.13 g, 1 mmol) or tetralin (0.12 g, 1 mmol),
respectively, was irradiated as a 10 mM solution in acetone
with 1 c (0.26 g, 1 mmol). After 5 h, the solvent was removed
in vacuo, and the reaction products were identified by coin-
jection with authentic samples. Using acetophenone as in-
ternal standard, the yields, which were based on the corre-
sponding starting materials, are as follows: indanone (95 mg,
0.72 mmol, 72%) and tetralone (104 mg, 0.71 mmol, 71%).
spectra were obtained in CDCl3 or D2O at
a 400 MHz
spectrometer. Analytical TLC was carried out on aluminum-
backed silica plates with detection by UV-light unless other-
wise noted.
Gen er a l P r oced u r e for th e P r ep a r a tion of Nitr op yr i-
d in iu m Sa lts 1a -c. A mixture of the corresponding pyridine
derivative (0.01 mol) and trimethyl- or triethyloxonium tet-
rafluoroborate, respectively (0.01 mol), in 80 mL dichlo-
romethane was stirred at room temperature for 4 h. The
solvent was evaporated in vacuo and the product recrystallized
from MeOH and ethyl acetate (4:1).
2-Hexanone (5 mL, 40.5 mmol) and 1b (49.2 mg, 0.2 mmol)
were dissolved in 15 mL of acetonitrile and irradiated for 5 h.
The solvent was evaporated and the residue analyzed by GC-
MS. The chromatogram showed only one product, which was
identified as 2,5-hexanedione by coinjection with the authentic
sample. The yield of 2,5-hexanedione (1.8 g, 0.02 mmol, 8%)
has been achieved using cyclohexanone as internal standard.
1-Meth yl-4-n itr op yr id in iu m tetr a flu or obor a te (1 a ):
yield 1.59 g (7.0 mmol, 70%); obtained as pale yellow needles;
2-Ethylnaphthalene (46.8 mg, 0.3 mmol) and 1c (78 mg, 0.3
mmol) were dissolved in 30 mL of acetonitrile and irradiated
for 12 h. The reaction mixture was worked up and quantified
as indicated above. The yield of 2-methylnaphthyl ketone (20.4
mg, 0.12 mmol, 12%) was determined using 1-phenylpentanone
as internal standard and based on the starting material.
1
mp 130 °C; H NMR (D2O, ppm) 4.56 (s, 3H), 8.73-8.80 (m,
2H), 9.25-9.27 (m, 2H). Anal. Calcd for C6H7N2O2BF4: C,
31.90%; H, 3.10%; N, 12.40%. Found: C, 31.97%; H, 3.11%;
N, 12.19%.
1-Meth oxy-4-n itr op yr id in iu m tetr a flu or obor a te (1 b):
yield 1.24 g (5.1 mmol, 45%); obtained as pale yellow needles;
Cyclohexane (30 mL, 0.28 mol) and 1b (0.25 g, 1 mmol) were
irradiated in 70 mL of acetone for 5 h. Acetone was cautiously
evaporated and the liquid residue identified by GC-MS-
analysis. The spectroscopic data of cyclohexanol, cyclohex-
anone, and bicyclohexyl corresponded in all respects with those
of the authentic samples. Quantification was achieved by
using cycloheptanone as internal standard. The yield of
cyclohexanol (31 mg, 0.84 mmol, 32%) was based on 2 mmol
of nitro-oxygen of 1b, whereas the yield of cyclohexanone (82
mg, 0.31 mmol, 41%) is based on 1 mmol of 1b.
1
mp 112 °C; H NMR (D2O, ppm) 4.47 (s, 3H), 8.36-9.42 (m,
2H), 9.55-9.63 (m, 2H). Anal. Calcd for C7H10N2O2BF4: C,
29.79%; H, 2.92%; N, 11.58%. Found: C, 29.66%; H, 2.96%;
N, 11.68%.
1-Eth oxy-4-n itr op yr id in iu m tetr a flu or obor a te (1 c):
yield 1.28 g (5.0 mmol, 46%); obtained as pale yellow needles;
mp 78 °C; 1H NMR (D2O, ppm) 1.47-1.54 (t, 3H) 4.49 (q, 2H),
8.89-8.96 (m, 2H), 9.56-9.64 (m, 2H). Anal. Calcd for
C8H12N2O2BF4: C, 32.85%; H, 3.54%; N, 10.94%. Found: C,
32.90%; H, 3.63%; N, 10.67%.
Gen er a l P r oced u r e for th e P h otoch em ica l Oxid a tion .
All photochemical reactions were run in a reactor equipped
with a high pressure 125 W Hg lamp. The compounds were
dissolved as indicated and placed in jacketed Pyrex vessels,
equipped with a stirbar and a septum. The resulting solutions
were cooled by a circulating water bath and degassed by
purging with N2 for 15 min. The reaction mixtures were
irradiated for the indicated time while stirring under N2. After
Isoamyl acetate (30 mL, 0.20 mol) was irradiated in 70 mL
of acetone with 1c (0.26 g, 1 mmol). After 5 h, acetone was
cautiously removed under reduced pressure and the liquid
residue added to a mixture of 1 mmol each of nitrobenzoyl
chloride and 4-(dimethylamino)pyridine in 4 mL of dichlo-
romethane and stirred 40 min while cooling at 0 °C. The
solvent was evaporated and the crude reaction product purified
by flash chromatography. The spectroscopic data of 17 and
of the nitrobenzoyl derivative of 17 have been compared with
(10) Masaki, Y.; Miura, T.; Mukai, I.; Iwata, I.; Oda, H.; Itoh, A.
Chem. Pharm. Bull. 1995, 43, 686-688.
(15) Severin, T.; Ko¨nig, D. Chem. Ber. 1974, 107, 1499-1509.
(11) Kharrat, A.; Gardrat, C.; Maillard, B. Can. J . Chem. 1984, 62,
2385-2390.
(16) Kellin, A. V.; Kulinkovich, O. Russ. J . Org. Chem. 1994, 30,
202-206.
(12) Ochial, E. J . Org. Chem. 1953, 18, 537, 548, 550.
(13) Simon, I. Chem. Zentralbl. 1929, 100, I, 2520.
(14) Bachmann, W. E.; Wendler, N. L. J . Am. Chem. Soc. 1946, 68,
2580.
(17) Rusell, G. A.; Kulkarni, S. V. J . Org. Chem. 1993, 58, 2678-
2685.
(18) Severin, T.; Adam, R.; Lerche H. Chem. Ber. 1975, 108, 1756-
1767.

Photooxidation of Hydrocarbons by Nitropyridinium Salts
J . Org. Chem., Vol. 63, No. 4, 1998 1143
the literature data.19,20 The yield of 17 (196 mg, 1.3 mmol,
67%) was based on 2 mmol of nitro-oxygen of 1 c, using
cyclohexanedione as internal standard.
THF (5 mL, 0.06 mol) and 1b (48.4 mg, 0.2 mol) or
nitrobenzene (24.6 mg, 0.2 mol), respectively, were irradiated
in 15 mL of acetone for 5 h. After evaporation of acetone and
the excess ether, the residual was treated with 2 N HCl and
extracted with dichloromethane. The combined dichloro-
methane layers were used in subsequent flash chromatography
with detection on silica plates using a mixture of vanillin (0.5
g), EtOH (20 mL), and H2SO4 (80 mL) and subsequent heating
to 120 °C. Beside 19 and 20, flash chromatography afforded
the C,C-coupling product 21, identified by comparison with
literature values.21 The oxidation products were quantified
as indicated above affording 19 (52 mg, 0.4 mmol, 92%), 20
(17.5 mg, 0.2 mmol, 100%), and 21 (24 mg, 0.2 mmol, 26%).
Using nitrobenzene for the irradiation, the yields are as
follows: 19 (43 mg, 0.3 mmol, 75%), 20 (3.3 mg, 0.04 mmol,
19%), and 21 (4.4 mg, 0.04 mmol, 5%).
THF (5 mL, 0.06 mmol) and 1b (48.4 mg, 0.2 mmol) or
nitrobenzene (24.6 mg, 0.2 mmol), respectively, were dissolved
in 15 mL of acetonitrile and irradiated for 5 h. The solvent
and the excess ether were removed in vacuo, and the liquid
residue was analyzed by GC-MS. The spectroscopic data of
19 and 20 corresponded in all respects with those of the
authentic samples. Quantification was achieved using cyclo-
hexanedione as internal standard. The yields were determined
on the assumption that the nitro group is photoreduced to a
hydroxylamine function. Photooxidation with 1b afforded 19
(16 mg, 0.1 mmol, 28%) and 20 (3.0 mg, 0.03 mmol, 17%).
Using nitrobenzene for the irradiation, the yields are as
follows: 19 (27 mg, 0.2 mol, 48%) and 20 (5.8 mg, 0.07 mol,
34%).
J O971617A
(19) Negele, S. Ph.D. thesis, Ludwig-Maximilians-Universita¨t
Mu¨nchen, Germany, 1996, p 240.
(20) J ulia, M.; Perez, C.; Sausinne, L. J . Chem. Res. M. 1978, 3401-
3418.
(21) Descotes, G.; Martin J .-C.; Labrit, G. Bull. Soc. Chim. Fr. 1969,
4151-4154.